The invention described herein was made or conceived in the course of, or under a contract with, the U.S. Department of Energy.
This invention relates to a method for doping semiconductors, and more particularly to a method for electrochemically diffusing a selected species of ions into a semiconductor substrate.
The valuable electronic properties of semiconductors are a result of precisely controlled amounts of impurities, or defects, which are introduced into the crystal structure of the semiconductor material. The processes by which these impurities are introduced are generally referred to as semiconductor doping. Numerous techniques for semiconductor doping have been developed as semiconductor technology has grown over the years. Perhaps the most common techniques have involved thermal diffusion of the impurity into the semiconductor. In thermal diffusion methods of doping the impurity is brought into contact with the semiconductor at an elevated temperature. Generally, the impurity is in the gas phase, but it may also be in the liquid phase. Both liquid and gas phase thermal diffusion methods suffer from the disadvantages that it is difficult to control the concentration, as a function of depth, of the impurity in the semiconductor and that relatively large amounts of energy are necessary to vaporize or melt the impurity.
Another method of doping semiconductors is to use accelerators, such as electrostatic accelerators, to generate energetic beams of ions of the desired impurity. These beams are then focussed on the semiconductor so that ions penetrate into the bulk semiconductor. While this method, generally known as ion implantation, does give better control of the concentration, it requires elaborate, expensive equipment and relatively large amounts of energy.
It is also known to electrochemically diffuse impurity ions from a liquid electrolyte, which ionically conducts the chosen impurity, into a semiconductor material.
For this method, however, the selection of impurities is limited, and the required low operating temperature limits the diffusion coefficient in the semiconductor, reducing the penetration of the impurity into the semiconductor.
In view of the above description of the known techniques for the doping of semiconductors and the inherent disadvantages of those techniques it is an object of the subject invention to provide a method for doping semiconductors which is capable of providing a high degree of control of the concentration of the impurity with depth.
It is another object of the subject invention to provide a method of semiconductor doping which does not require large amounts of energy to vaporize or melt bulk quantities of the impurity.
It is still another object of the subject invention to provide a method of semiconductor doping which does not require elaborate and expensive equipment such as ion accelerators.
It is also an object of the subject invention to provide a method of semiconductor doping which is capable of use over a broad class of impurities and semiconductors.
It is also an object of the subject invention to provide an electrochemical diffusion process which may be carried out at high temperatures.